Ferrofluid is a stable suspension of magnetic particles in a liquid carrier. The particles, which have an average size of about 10 nm, are coated with a surfactant which prevents the particles from agglomerating, even when a strong magnetic field gradient is applied to the ferrofluid. A typical ferrofluid may contain by volume 5% magnetic solid, 10% surfactant and 85% carrier. According to Ferrotec (USA) Corporation, a manufacturer of ferrofluids, a wide variety of magnetic solids, surfactants, and carriers are available, which permits a user to tailor the ferrofluid's properties based on the specific application.
Regardless of their composition, ferrofluids generally behave the same. In the absence of a magnetic field, the magnetic moments of the particles in the ferrofluid are randomly distributed and the fluid has no net magnetization. When a magnetic field is applied to a ferrofluid, the magnetic moments of the particles orient along the field lines. Ferrofluids typically respond almost immediately to changes in the applied magnetic field and when the applied field is removed, the moments randomize again quickly. In a gradient field the whole fluid responds as a homogeneous magnetic liquid which moves to the region of highest flux. This means that ferrofluids can be precisely positioned and controlled by an external magnetic field. The retention force of a ferrofluid can be adjusted by changing either the magnetization of the fluid or the magnetic field in the region. Additional information regarding ferrofluids can be found at http://en.wikipedia.org/wiki/Ferrofluid, which is incorporated herein by reference in its entirety.
Among their various uses, ferrofluids can be used as seals. By way of example, ferrofluids are used to seal rotary moving parts since the distance between the moving parts does not change, and the fluid can remain locked inside the seal by a constant magnetic force. This is described in more detail at http://www.roymech.co.uk/Useful_Tables/Seals/Rotary_Seals.html, which is incorporated herein by reference in its entirety. However, in applications where the distance between the parts to be sealed changes significantly, the magnetic force will not remain constant, and the fluid will not remain captured in place.
What is needed is an apparatus through which a ferrofluid can be used to selectively seal off a space without the ferrofluid dissipating.
Sealing of a linearly movable assembly is often accomplished with compression seals or gaskets. When the linear motion is accomplished by a spring or springs, however, the sealing force imparted by the spring(s) is at a minimum at one end of the range of motion. If a seal is desired at this position, the spring force may not be sufficient to provide a good seal. If the spring force is increased to overcome this situation, this results in additional forces when the spring is compressed, which can be a serious disadvantage to this sealing method. The invention described herein overcomes this disadvantage by eliminating reliance on the spring force to create a seal.